The bandpass filter is in a simplest form composed of a plurality of resonators arranged in such a way that the input signal is put to an input port, then to the first resonator and then passes sequentially second and other resonators until it reaches the last resonator and leaves the filter at an output port.
Radio Frequency, RF, bandpass filters in e.g. a base station use air cavity coaxial resonators. This technology is applied to build filter used in base stations that handle moderate and high power levels between 10-120 W RF power at frequencies about 500-3000 MHz.
Dielectric resonators are used to shrink the filter size and obtain higher Q value in bandpass filters. The Q value is a quality factor of a resonator, e.g. ratio between stored energy and dissipated energy within the resonator. Different dielectric resonators with different operating mode are used. Some solutions use Transverse Electric, TE, mode that has very high Q value and moderate size reduction.
Another example is to use a Transverse Magnetic, TM, mode dielectric resonator. It offers higher Q in the same volume and good power handling capability. The advantage with TM-mode technique is explained in detail in Ericsson U.S. Pat. No. 8,773,222 B2.
Today's market offers a range of high dielectric material with dielectric permittivity from 10 to 48 with sufficient RF properties such as Q value and thermal expansions coefficient with a reasonable manufacturing cost. These types of dielectric materials have been used in filter with TE mode and TM mode for frequency band above 1 GHz.
The previous technologies have size problems at low frequencies below 1 GHz thus hampering smaller base stations.